Add cold water into a bowl. Submerge ox tails and let it soak for about 2hours. Change water every 30min.
Bring water to boil in a big stock pot. Once it boils, add the ox tail, making sure it's almost submerged. Boil for 5-7min. Discard this water and wash the ox tail in cold water. Set to drain.
Take the same pot and bring about 18-20 cups of water to boil. Once the water boils, add ox tail and simmer in medium heat for 1 hour.
Skim the water and add 12 more cups of fresh water. Bring to broil and then simmer in medium heat for another 1.5 hour.
Add onion, garlic, ginger, scallion roots and continue to simmer in medium heat for 1.5 hour. Turn off fire, take out all the aromatics.
Serve with sea salt, black pepper, and chopped scallions on the side in individual bowls.